1. Field of the Invention
The present invention relates to an error correction optical communication system having a forward error correction (FEC) function, and, more particularly to an error correction optical communication system that transmits and receives a non-interleaved information frame.
2. Description of the Related Art
An optical access system, a so-called “Fiber To The Home (FTTH)”, that transmits large-capacity information to houses and offices, is being distributed rapidly. Among others, the service of “Gigabit Ethernet (registered trademark, and hereinafter referred to as “Ethernet®”) PON (normally known as GE-PON)” that uses a “Passive Optical Network (PON)” system is being spread rapidly as a method of connecting Gigabit Ethernet® between a telecommunications house and plural users, since specifications are standardized in the IEEE Std 802.3ah.
The GE-PON has a configuration that an “Optical Line Terminal (OLT)” as a station-side device and an “Optical Network Unit (ONU)” as a user-side device are connected in two directions with one optical fiber via an optical branch unit. A point-to-multipoint connection, for example, a connection between one OLT and 32 ONUs, is made possible by carrying out a burst transmission and reception in which a time slot is shared among users.
According to the GE-PON, the optical branch unit branches power. Therefore, the optical power that each ONU receives is attenuated to one to the number of branches, and light that the OLT receives from each ONU is also attenuated to one to the number of branches. Consequently, a bit error is likely to occur. Furthermore, the fact that a laser diode having low performance is being used to decrease the cost is also likely to cause this bit error problem. To solve these problems, each of the OLT and the ONU is equipped with the FEC function, and a system of correcting bit errors whose amount is smaller than that the FEC can correct is standardized in the IEEE Std 802.3ah.
The FEC prescribed by the “IEEE Std 802.3ah.” is Reed-Solomon (255, 239). According to this FEC, 16-byte error correction symbols (hereinafter, “FEC parity”) are added to 239-byte information data symbols, thereby structuring a block of “239+16=255” bytes. When the Ethernet® data is smaller than 239 bytes, “zeros” are filled in the data to satisfy 239 bytes. A starting sequence and an ending sequence are added before and after the FEC parity to be added. When the system does not use the FEC, the added FEC parity is disregarded, thereby carrying out communications without changing the conventional device.
The Reed-Solomon (255, 239) error correction system has a capacity to be able to correct up to eight byte errors. In other words, the Reed-Solomon (255, 239) error correction system can correct all bit errors when the error is within eight bytes among 255 bytes. However, when a bit error occurs extending to nine bytes, the Reed-Solomon (255, 239) cannot correct the error. In a transmission path of actual optical communications, bit errors occur continuously in some cases, due to a fluctuation of polarization, non-linearity of an optical fiber, or insufficient performance of a transmitter/receiver. Even when the total number of bit errors that occur during a predetermined time is equal to or smaller than the number of bit errors that the Reed-Solomon (255, 239) error correction system can correct, the errors cannot be corrected when these bit errors are burst errors that occur during a short period of time. To solve this problem, there is a method of randomizing the occurrence of bit errors so that the bit errors do not occur in burst. This method is disclosed in the ITU-T Recommendation G.975, for example.
According to the FEC system disclosed in the ITU-T Recommendation G.975, an FEC encoder adds the FEC parity to a transmission information frame, and thereafter, an interleaver changes the order of bits, at the transmission side. On the other hand, at the reception side, a process opposite to that carried out at the transmission side is carried out. In other words, a de-interleaver and an FEC decoder are used to reproduce the transmission information frame. According to the ITU-T Recommendation G.975, the bit order is changed within 16 codewords. For example, when a continuous burst error of 512 bits occurs in a transmission path, 16 codewords are returned to an original bit string in a de-interleaver at the reception side, and the error is input as a continuous error of 32 bits (obtained by 512/16), to the FEC decoder. In this case, the continuous 32 bits become equal to or smaller than five bytes. Therefore, the Reed-Solomon (255, 239) error correction system can correct all errors.
In the “IEEE Std 802.3ah” document, the standard GE-PON device has both systems using the FEC and the system not using the FEC. Therefore, an interleave operation of the Ethernet® data series is not carried out. This is because when the interleave of the Ethernet® data series is carried out, a system that does not have a de-interleaver (not using the FEC) at the reception side cannot receive the data. Therefore, according to the Reed-Solomon (255, 239) error correction system that does not carry out interleaving of the Ethernet® data series, even when one bit error occurs for each nine bytes within one block in the Reed-Solomon (255, 239), this block cannot be corrected in the worst case. Consequently, the Ethernet® packet is discarded by an Ethernet® frame check sequence. In other words, the burst error tolerance is considerably low.
On the other hand, systems called a junction code and a product code, each constituted by two or more codewords, are known as a method of increasing the error correction capacity. However, each codeword needs to be interleaved, and there arises a problem that a decoding cannot be achieved correctly by merely disregarding an added FEC parity.